Lithographic sheet material in commercial use nowadays typically comprises an aluminum alloy substrate that must be grained and anodized before it is coated with a photosensitive layer. "Graining" refers to a mechanical or chemical process for roughening the aluminum sheet surface. The process results in a substrate surface having a uniform coarse texture that will provide adequate resolution in the prints to be made, while lasting through thousands of printings before the sheet must be replaced in a printing press. Anodizing the grained sheet then provides a desired microstructure on the aluminum surface for print durability.
Although grained and anodized lithographic sheet material performs adequately, it is difficult and expensive to produce. Mechanical graining requires the use of an abrasive slurry and brushes with operations controls necessary to account for brush wear. Chemical graining requires treatment with strong basic or acidic etching solutions which are difficult to dispose of properly. Anodizing requires expenditures of large amounts of electrical energy in its operation and also generates effluents that must be carefully handled to avoid environmental problems. Electrochemical graining and anodizing require expenditures of large amounts of electrical energy. Graining and anodizing also generate effluents that must be handled carefully to avoid environmental problems. Because of difficulties inherent in the prior art graining and anodizing processes, persons skilled in the art have come to recognize that it would be desirable to eliminate both graining and anodizing in the commercial production of lithographic sheet material. However, no completely desirable alternative was developed prior to the present invention.
A principal objective of the present invention is to provide a lithographic sheet material and a process for its production which avoid the graining and anodizing steps conventionally utilized for making lithographic sheet.
A related objective of the invention is to provide a lithographic sheet production process that minimizes incidental generation of waste material requiring disposal.
A further objective of the invention is to provide a lithographic sheet material that can be manufactured economically, using readily available and environmentally acceptable starting materials.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description.